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package jdk.internal.jimage.decompressor;

import java.io.DataInputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.ArrayList;
import java.util.List;

Index compressor. Use the minimal amount of bytes required to store an integer.
Implementation Note:This class needs to maintain JDK 8 source compatibility. It is used internally in the JDK to implement jimage/jrtfs access, but also compiled and delivered as part of the jrtfs.jar to support access to the jimage file provided by the shipped JDK by tools running on JDK 8.
/** * * Index compressor. Use the minimal amount of bytes required to store * an integer. * * @implNote This class needs to maintain JDK 8 source compatibility. * * It is used internally in the JDK to implement jimage/jrtfs access, * but also compiled and delivered as part of the jrtfs.jar to support access * to the jimage file provided by the shipped JDK by tools running on JDK 8. */
public class CompressIndexes { private static final int COMPRESSED_FLAG = 1 << (Byte.SIZE - 1); private static final int HEADER_WIDTH = 3; private static final int HEADER_SHIFT = Byte.SIZE - HEADER_WIDTH; public static List<Integer> decompressFlow(byte[] values) { List<Integer> lst = new ArrayList<>(); for (int i = 0; i < values.length; i += getHeaderLength(values[i])) { int decompressed = decompress(values, i); lst.add(decompressed); } return lst; } public static int readInt(DataInputStream cr) throws IOException { // Get header byte. byte header = cr.readByte(); // Determine size. int size = getHeaderLength(header); // Prepare result. int result = getHeaderValue(header); // For each value byte for (int i = 1; i < size; i++) { // Merge byte value. result <<= Byte.SIZE; result |= cr.readByte() & 0xFF; } return result; } private static boolean isCompressed(byte b) { return (b & COMPRESSED_FLAG) != 0; } private static int getHeaderLength(byte b) { return isCompressed(b) ? (b >> HEADER_SHIFT) & 3 : Integer.BYTES; } private static int getHeaderValue(byte b) { return isCompressed(b) ? b & (1 << HEADER_SHIFT) - 1 : b; } public static int decompress(byte[] value, int offset) { // Get header byte. byte header = value[offset]; // Determine size. int size = getHeaderLength(header); // Prepare result. int result = getHeaderValue(header); // For each value byte for (int i = 1; i < size; i++) { // Merge byte value. result <<= Byte.SIZE; result |= value[offset + i] & 0xFF; } return result; } public static byte[] compress(int value) { // Only positive values are supported. if (value < 0) { throw new IllegalArgumentException("value < 0"); } // Determine number of significant digits. int width = 32 - Integer.numberOfLeadingZeros(value); // Determine number of byte to represent. Allow for header if // compressed. int size = Math.min(((width + HEADER_WIDTH - 1) >> 3) + 1, Integer.BYTES); // Allocate result buffer. byte[] result = new byte[size]; // Insert significant bytes in result. for (int i = 0; i < size; i++) { result[i] = (byte)(value >> ((size - i - 1) * Byte.SIZE)); } // If compressed, mark and insert size. if (size < Integer.BYTES) { result[0] |= (byte)(COMPRESSED_FLAG | (size << HEADER_SHIFT)); } return result; } }